Called this ~4 years ago when they were acquired by M$.

But it's still really sad to see. I use it because it was trivial to CSS style it to match my Desktop. It's comfortable to me in a way that other editors thus far have not been.

Are there other editors with this level of customizability? I know VSCode and Sublime support theming but from what I can tell it involves installing pre-packaged themes.

If level of customizability is your main metric:

Emacs can go from this: https://emacshints.files.wordpress.com/2013/09/spash1.png

to this: https://github.com/rougier/nano-emacs